The present invention relates to a control device for the timed control of the filling pressure during the filling of a press die with a pasty or slip-like mass, especially for the production of oxide magnets, according to the pre-characterizing clause of the main claim.
To ensure fault-free manufacture, an important aim of production operations of this type is to keep the press thickness and press density of the magnet blanks within narrow tolerance limits, whilst at the same time as rapid a production cycle as possible should be sought after. For this, after the maximum filling pressure has been reached, it is necessary to maintain this for a set predeterminable time. If this time is too short, there is the danger that the production quality will be diminished, and if it is too long there is a needless lengthening of the filling operation.
A control device of the above mentioned generic type is known from German Offenlegungsschrift 3,347,035. The aim of this, when the maximum filling pressure is reached, is to cut in a timing element which maintains the filling pressure during its holding time. This is achieved because, when a desired pressure value is reached during this holding time, the filling pressure is regulated to the desired value. However, a hydraulic regulating device involving a high outlay is required for this purpose. In contrast, if a regulating device were omitted and the pump pressure predetermined as the maximum filling pressure, the triggering of the holding time as a result of a desired-value comparison is too unreliable, since the desired value for the maximum filling pressure is possibly not reached as a result of pressure fluctuations. In order nevertheless to conduct a filling operation without a pressure-regulating circuit, a holding time is therefore predetermined for the entire filling pressure via a timing element in a likewise known way. But the disadvantage of this is that the flow time and filling-pressure holding time are added to the total holding time, and therefore in the event of a lengthening of the flow-pressure time as a result of fluctuations in the flow conditions and in the consistency of the flowing mass the filling-pressure holding time could be reduced in an inadmissible way. The opposite situation, namely a lengthening of the filling-pressure holding time as a result of a reduction of the flow time, once again leads to a needless lengthening of the production cycle. Because of these fluctuations, it is also necessary to equip the total holding time with a safety reserve, thus once more leading to a needless lengthening of the production cycle.